1. Field of the Invention
The present invention relates to a copper foil for printed circuit board with taking environmental conservation into consideration, and more particularly to a copper foil for printed circuit board with taking environmental conservation into consideration which does not contain hexavalent chromium, and has good heat resistance, moisture resistance and bondability with respect to a base material.
2. Description of the Related Art
A copper foil or a copper alloy foil (hereinafter, referred to simply as “copper foil”) is widely used for the purpose of a conductor (conductive member or conductive strip). Particularly in the field of the flexible printed circuit (FPC), a printed circuit board is manufactured by layering (laminating) a copper foil on a polyimide film or by coating a copper foil with a varnish mainly composed of polyamic acid. Hereinafter, materials such as polyimide film, varnish, or solidified varnish to be used for the printed circuit board are referred as “base material (substrate) for a printed circuit board” or simply as “base material”.
A good bonding is required between the copper foil and the base material for a printed circuit board. Therefore, the roughening treatment is frequently conducted for an bonding surface of the copper foil to increase an anchoring effect, thereby improving the bonding with the base material for a printed circuit board.
The copper foil is classified into an electro-deposited copper foil and a rolled copper foil according to the manufacturing method therefor. However, the roughening treatment is conducted in similar manner for these two types of copper foils. For example, as a manner of roughening treatment, a manner of applying (depositing) copper in the form of rice-grains on a surface of the copper foil by burnt plating and a manner of selectively etching grain boundaries by using acid are generally used.
As to the roughening treatment using the burnt plating, the roughening treatment by alloy platings has been developed in addition to the usual copper plating. Japanese Patent Laid-Open (Kokai) No. 52-145769 (JP-A-52-145769) discloses copper-nickel alloy plating as a representative example of alloy platings.
In addition, as to a surface treatment after such a roughening treatment, Japanese Patent Publication for Opposition (Kokoku) No. 6-54829 (JP-B-6-54829) proposes to provide a cobalt plating or cobalt-nickel alloy plating, etc.
On one hand, as to a method for improving the bonding with the base material, there is a method for providing an anchoring effect by using the roughening treatment (improvement in physical bondability), as described before. Further, there is also a step for improving a chemical bondability between the copper foil and the base material for a printed circuit board by a surface treatment for providing a metallic layer with a high affinity with the base material on a surface of the copper foil.
Conversion treatment such as so-called chromate treatment or silane-coupling treatment conducted for a surface of the copper foil is an example of the method for providing the metallic layer with the high affinity with the base material on the copper foil surface. The chromate treatment and silane-coupling treatment are methods for the purpose of rust preventing the surface of the copper foil as well as for the purpose of improving the bondability with the base material for a printed circuit board, as disclosed in Japanese Patent No. 3142259 and Japanese Patent Laid-Open (Kokai) No. 2005-8972 (JP-A-2005-8972). In addition, the rust prevention (corrosion resistance, oxidation resistance) effect is expected (required) also for the surface of the copper foil, which is a surface not to be bonded with the base material (the back side of the bonding surface).
As disclosed in the JP-B-6-54829, Japanese Patent No. 3142259, and JP-A-2005-8972, the chromate treatment has been conducted by immersing the copper foil to be treated in a treatment solution containing hexavalent chromium, or by electrolyzing the copper foil which is provided as an anode or a cathode in the chromate treatment solution. However, considering the recent emphasis on the environmental protection, the conversion treatment free of any hexavalent chromium has been developed.
As one of techniques therefor, a chromate treatment wherein a treating solution containing trivalent chromium is used exhibits the most remarkable actual achievement, and it becomes commercially available as shown in, for example, JP-A-2005-42139.
In reality, however, hexavalent chromium-free plating does not proceed in a manufacturing process of a copper foil, because its rustproof ability is usually inferior to a chromate treatment wherein a treating solution containing hexavalent chromium is used as compared with that of hexavalent chromium-free treating solution, besides there is a burden share for changing facilities.
In addition, there is another reason for preventing progress in a trivalent chromium treatment in that a coating weight of hexavalent chromium per unit area in a copper foil used in a printed circuit board is essentially very small, so that it is difficult to detect as a result of analysis.
According to a chromate treatment wherein a conventional treating solution containing hexavalent chromium is used, however, it cannot be denied that there is a possibility of providing totally a significant load to environment, even if it is a very small amount of coating weight in a present situation where a demand of a copper foil increases from year to year.